In recent years, with the spread of information terminals, there are increasing demands for a flat panel display that serves as a display for a computer. Further, with development of the information technology, there has been increased a chance for information offered in a form of a sheet of paper medium in the past to be offered in an electronic form. An electronic paper or a digital paper is demanded increasingly as a display medium for a mobile that is thin, lightweight and handy.
In the case of a display device of a flat sheet type, a display medium is generally formed using an element that employs a liquid crystal, organic EL or electrophoresis method. In the display medium of this kind, a technology for using an active driving element comprised of a thin-film transistor (TFT), serving as an image driving element, is the main current for ensuring uniform image brightness and an image rewriting speed.
A TFT is ordinarily manufactured by a process comprising forming, on a glass substrate, a semiconductor layer of a-Si (amorphous silicone) or p-Si (poly-silicone) and metal films of source, drain and gate electrodes, in the order. In the manufacture of a flat panel display employing such a TFT, a photolithography step with high precision is required in addition to a thin layer forming step requiring a vacuum line carrying out a CVD method or a sputtering method or a high temperature treatment step, which results in great increase of manufacturing cost or running cost. Recent demand for a large-sized display panel further increases those costs described above.
In order to overcome the above-described defects, an organic thin-film transistor employing an organic semiconducting material has been extensively studied (see, for example, Japanese Patent O.P.I. Publication No. 10-190001 1 and “Advanced Material”, 2002, No. 2, p. 99 (review)). Since the organic thin-film transistor can be manufactured at low temperature employing a lightweight substrate difficult to be broken, a flexible display employing a resin film as a substrate can be realized (see, for example, SID '02 Digest P. 57). Further, employing an organic semiconducting material allowing a wet process such as a printing method or a coating method, a display manufacturing process, which provides excellent productivity and reduced cost, is expected to be realized.
A conventional manufacturing process of a TFT or a TFT sheet comprises forming a film of a metal such as chromium, nickel or aluminum according to a sputtering method, and processing the film according to photolithography including etching to form a gate busline or a gate electrode. However, this process has problems in that gate leakage is likely to occur which is caused by minute unevenness of the electrode surface, unevenness (herein also referred to as hillocks) of a layer adjacent to the electrode, which deteriorates smoothness of the electrode surface, or edge portions of the electrode.
Further, there is problem that a TFT or a TFT sheet employing a resin sheet substrate, in which unevenness of the sheet surface deteriorates smoothness of the electrode surface, is likely to leak from the gate busline or the gate electrode, as compared with a conventional TFT or TFT sheet employing a glass plate having a smooth surface.